A method for producing a motor casing having a first casing member and a second casing member which are separated in a direction of a rotation shaft of a motor is disclosed. The first and second casing members are coupled by coupling members, and a stator of the motor is held by the first casing members and the second casing member to be fixed within the casing. The method is characterized in that when it is assumed that a length of the stator held is L1, and a length from an engaging portion of the coupling member with the first casing member to an engaging portion of the coupling member with the second casing member is L2, a strength classification of the required coupling member is determined in advance by converting it into a ratio of L1 and L2.